The term “birefringence” means that different linear polarizations of light travel at different speeds through light-transmissive optical material. Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam traversing the sample. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient, however, to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm).
Oftentimes, the term “birefringence” is interchangeably used with and carries the same meaning as the term “retardance.” Thus, unless stated otherwise, those terms are also interchangeably used below.
U.S. Pat. Nos. 7,385,696 and 6,473,179, herein incorporated by reference, disclose systems for the precise measurement of birefringence in various optical materials. When so measured, the optical material is often referred to as a “sample.” An important component of such systems includes a resonant polarization modulation device in the form of a photoelastic modulator or PEM.
In one approach, as illustrated and described in U.S. Pat. No. 6,473,179, the system for precisely measuring low-level birefringence properties of optical materials incorporates a single PEM for modulating a beam of polarized light that is then directed through a sample. The beam propagating from the sample is separated into two parts, with one part having a polarization direction different from the polarization direction of the other beam part. These separate beam parts are then processed as distinct channels. Detection mechanisms associated with each channel detect the time-varying light intensity corresponding to each of the two parts of the beam. The information is combined for calculating among other things, a precise measure of the retardance induced by the sample.
Another approach (as exemplified in an embodiments disclosed in U.S. Pat. No. 7,385,696) uses an optical setup that includes two PEMs to measure linear birefringence. This setup will be hereafter referred to as a dual PEM setup. This system of this embodiment can determine birefringence properties of optical materials such as polymeric films, as well as single-crystal materials such as quartz, calcite, mica, and sapphire. The birefringence of interest may be intrinsic to the material or induced by external forces.
The dual PEM setup generally comprises three modules. The top module includes a light source, a polarizer oriented at 45 degrees, and a PEM oriented at 0 degrees. The bottom module includes the second PEM that is set to a modulation frequency that is different from the modulation frequency of the first PEM. The second PEM is oriented at 45 degrees. The bottom module also includes an analyzer at 0 degrees and a detector. The middle module includes a sample support, which can be any of a variety of mechanisms for supporting a sample, such as polymeric film, etc., in position between the top and bottom modules to allow a light beam from the source of the setup to pass through the sample as described more below. The sample support may be of a type that mounts to a computer-controlled, movable X-Y stage to allow the sample to be scanned by the light beam across the area of the sample.
As a single beam is directed through a central optical aperture in the PEM, either the sample or the optical setup is moved so that the sample is scanned by the beam to enable multiple discrete measurements to be taken across the area of a sample to detect and graphically display variations in the birefringence properties across the sample area.